1. Field of the Invention
The present invention relates to a semiconductor device and, in particular, to a protection diode for protection against a possible negative surge voltage which is applied to a bonding pad.
2. Description of the Related Art
In a semiconductor device, such as an IC and LSI, a plurality of bonding pads 13 formed of, for example, aluminum are formed on the marginal area of a semiconductor chip 11 as shown in FIG. 1. The pads are connected to a semiconductor device area 12 by a wiring, such as an aluminum, and connected to lead terminals (not shown) by bonding wires (not shown) so that the semiconductor device area 12 is electrically connected to the lead terminals. A negative surge voltage protection diode 14 is formed between the pads 13 as shown in FIGS. 2A and 2B to allow a negative surge voltage which is induced from the lead terminal to be grounded. It is required that the diode has the surge withstand voltage of about -250 V. The aforementioned diode is manufactured as will be set forth below. A buried layer 15 of first conductive type (N type) having a high impurity concentration is formed, by thermal diffusion, in a protection diode formation area of the P type substrate 22. A first area 21 of N conductivity type lower in impurity concentration than the buried layer 15 is epitaxially grown on the substrate surface. An isolation layer 24 is thermally diffused around a pad formation area in the first area 21 and around a negative surge voltage protection diode 14 in the first area to achieve an element-to-element isolation in which case the isolation layer extends down to the substrate surface. When this is done, an island area and protection diode island area are formed beneath the pad. Then an N type diffusion region 16 and P type diffusion region 17 near to the diffusion region 16 are formed on the island area of a protection diode, serving as a cathode and anode, respectively. In this way, an insulating oxide film 25 is formed on the surface of the N type first area 21, isolation layer 24, N type diffusion region 16 and P type diffusion region 17. Of the oxide film 25, those portions overlying the N type diffusion layer 16 and P type diffusion layer 17 are etched to form a hole in the diffusion regions 16 and 17. Aluminum is evaporated over the pad formation area and over the respective diffusion layers to provide a pad 13 extending up to the N type diffusion region 16 as well as an electrode 18 connected to the P type diffusion layer 17.
In the semiconductor device thus manufactured, the protection diode 14 comprises the diffusion region 16 electrically connected to the pad 13 and diffusion region 17 connected to the electrode 18. Upon the generation of a negative surge current from the lead terminal to the pad 13, it flows into the N type diffusion region 16. Since the N type diffusion region and P type diffusion region provide a PN junction, the electric current flows into the P type diffusion region 17 and a potential is taken out of the pad via the electrode 18. Upon applying a surge voltage between, for example, the voltage 18 and a ground terminal no surge current flows across the semiconductor area 12, thus preventing a breakage of the semiconductor device.
In the aforementioned conventional semiconductor device, the diode area including the N type region 16 and P type region 17 has to be secured independently of the pad area because a withstand voltage of about -250 V is necessary. That is, an island area which is secured for the protection diode requires a third to a half of the island area underlying the pad 13, thus offering a bar to a reduction in a pad-to-pad distance and hence to an improvement in an integration density.